1. Field of the Invention
The present invention relates to a technique for detecting a position of a substrate and a technique for correcting the position of the substrate, in a processing vessel for providing a process, such as a plasma etching process and/or CVD process, to the substrate, for example, a semiconductor wafer.
2. Background Art
In a process for manufacturing a flat panel, such as a semiconductor device or liquid crystal display device, a substrate processing apparatus, such as a plasma etching apparatus or CVD apparatus, is used to provide a process, including etching and/or film forming, to a substrate, such as a semiconductor wafer (hereinafter, referred to as a wafer) or glass plate.
Hereinafter, a parallel flat plate type plasma etching apparatus for providing a plasma process to a wafer will be described by way of example. The plasma etching apparatus of this type includes a processing vessel adapted to provide a process to a wafer, a wafer placing table provided in the processing vessel and used also as a lower electrode, and a shower head provided above the placing table and constituting an upper electrode. Vacuum is applied into the processing vessel by using a vacuum pump, and high frequency voltage for generating plasma is then applied to the placing table by using a high frequency power source, so as to generate plasma between the placing table and the shower head, whereby a processing gas introduced into the processing vessel from a gas supply chamber can be activated by the plasma. Thus, an etching process due to such plasma is provided to the wafer placed on the placing table.
Upon mounting the wafer on the placing table, the wafer is carried into the processing vessel from the outside, by using a carrying apparatus, and the carrying apparatus is then stopped in a predetermined transferring position, and thereafter three lifting pins (transfer means) are projected out from a top placing face of the placing table, so as to transfer the wafer onto the lifting pins. On the periphery of the top face of the placing table, a ring-like member, which is called focus ring, is provided for controlling the plasma, wherein the wafer is placed in a predetermined position (hereinafter, referred to as a placing position) in a recess surrounded by the focus ring. Thereafter, when the lifting pins holding the wafer thereon are lowered, the wafer is transferred to the placing position.
In a semiconductor manufacturing factory, a multi-chamber system is often used as a vacuum processing apparatus, such as a plasma etching apparatus or the like, in which a plurality of vacuum chambers (processing vessels) are airtightly connected with a carrying chamber, which is also used as a vacuum chamber and includes a carrying apparatus therein. In this case, for example, by utilizing an alignment mechanism including a rotary stage and an optical sensor in the carrying chamber, the wafer can be held accurately in a predetermined position where the wafer is to be transferred onto the lifting pins from the carrying apparatus. In the carrying apparatus, however, errors tend to occur in driving systems due to stretch or the like of driving belts provided in the carrying apparatus, and/or deformation is likely to occur in each chamber due to pressure to be applied to the large-sized carrying apparatus by the application of vacuum. Because of these factors, the central position of the wafer may be shifted from the central position of the placing table when the wafer is located on the placing table. For example, in the case of carrying out processes, in succession, by carrying the wafer to one processing vessel after another in the multi-chamber system, since errors between the carrying apparatus and the wafer are accumulated, a significant positional error or difference may occur between the wafer transferring position for transferring the wafer from the carrying arm onto the lifting pins and the placing position of the placing table. Therefore, if continuing such a mounting operation of the wafer while leaving the positional error as it is, the wafer may tend to ride on the focus ring surrounding the wafer placing position, thus making it difficult to perform a normal process, and the placing table which is not completely covered due to such a centered-off wafer may tend to be exposed to the plasma and thus damaged therewith. In addition, such improper mounting of the wafer onto the placing position (i.e., the positional error between the respective central positions) may also lead to having negative impact on the yield of products. Furthermore, in assessment on the process provided to the wafer, for example, in assessment on a profile of the etching rate in the surface of the wafer, factors to be attributed to the process parameters can not be distinguished from factors to be attributed to the aforementioned positional error of the wafer, as such making it difficult to analyze obtained data correctly and to carry out assessment accurately.
Moreover, because vacuum chuck can not be utilized in the vacuum processing apparatus, such as a plasma etching apparatus, as described above, the wafer is held by the placing table by using an electrostatic chuck utilizing coulomb force or Johnson-Rahbek force. The fixation of the wafer due to such electrostatic chuck is released after completion of the etching process. However, an electric charge may tend to remain in the electrostatic chuck even after the so-called de-chucking. If attempting to raise the lifting pins and transfer the wafer to the carrying apparatus in such a state, the wafer may tend to leap upward due to strong force to be applied by the lifting pins or otherwise be raised up obliquely. This is called de-chucking error. Furthermore, if continuing operation of the carrying apparatus, irrespectively of occurrence of such a de-chucking error, failures of transfer of the wafer to the carrying apparatus and/or falling of the wafer from the carrying apparatus due to its collision with a transfer port of each processing vessel may occur.
To address these problems, Patent Document 1 describes a technique for detecting a current position of a wafer held by a carrying apparatus by using a photo-sensor, by attaching the photo-sensor for detecting the position of the wafer to a placing face of a placing table and/or a wall portion of each processing vessel. According to this technique, if the current position of the wafer is shifted relative to a position in which the wafer transfer is to be carried out (i.e., a position in which the central position of the wafer is coincident with the central position of the placing table, and this position will be referred to, hereinafter, as a programmed position), the transfer operation can be carried out after the wafer transferring position has been corrected by moving the carrying means.
However, in the Patent Document 1, the photo-sensor of a type to be attached to a wall portion of each processing vessel is configured to detect the wafer position based on whether sensor light emitted in an oblique direction is blocked by the wafer or not. Therefore, the shading area of the sensor light beam is significantly large, thus making it difficult to accurately measure fine differences. In addition, a relatively long optical path of the sensor light renders adjustment of attaching positions of a light-emitting sensor and a light-receiving sensor significantly difficult. Furthermore, deformation of the placing table to be caused by multiple exposure processes under high temperature conditions due to plasma may substantially shift the placing position of the wafer. However, since the photo-sensor attached to the processing vessel is adapted to confirm only the wafer position, it can not be used to correct the positional error caused by factors on the side of the placing table.
On the other hand, the photo-sensor of a type to be attached to the placing table grasps the wafer position by irradiating the periphery of the wafer with light and detecting the reflected light. Accordingly, the photo-sensor is attached to the top face of the placing table while being projected out the periphery of the wafer. Therefore, in some cases, the photo-sensor can not be completely covered with the wafer even after the wafer is placed on the placing table. In such a case, a part of the photo-sensor, which is not covered with the wafer, is exposed to plasma, and is hence likely to be damaged. Moreover, the placing table used in the plasma etching apparatus has a complicated structure because it includes an electrostatic chuck for fixing the wafer and a fluid passage for supplying a back side gas for enhancing heat conductivity between the placing face and the wafer rear face, in addition to the lifting pins described above. Accordingly, these elements should make the structure of the placing table more complicated and thus impractical.
In the above Patent Document 1, while a method for correcting the positional error, upon carrying the wafer into the plasma etching apparatus, by using the carrying apparatus is described, no measures for addressing the positional error associated with the de-chucking error are described.    Patent Document 1: TOKUKAIHEI No. 7-201952, KOHO: page 3˜, paragraphs [0009] to [0015]